Immunohistology of the retro-orbital centroblastic B-cell lymphoma arising in monkey 4. Sections were stained with hematoxylin-eosin (A) and immunostained with anti-CD20 MoAb (B). Immunohistochemical and in situ hybridization (brown) double-staining of CD20⁺ cells (yellow ring) (C) and of CD68⁺ cells (orange ring) (D) displaying intracellular SIV genome signals (dark brown).